Convert NSDictionary values to NSString

I have an NSDictionary in which the values are of different types, but need to be converted to strings and put back into the original NSDictionary. I have the following so far:

for (id key in myDictionary) {
    id value = [myDictionary objectForKey:key];
    value = (NSString *)[NSString stringWithFormat:@"%@", value];
}

However this is not working - when I print out the type of each value afterwards, they remain the same as before. Can anyone advise?